Superconductor and method of producing same

ABSTRACT

A high critical temperature and high critical current density superconductor is disclosed which contains a metal oxide expressed by the following formula (I): 
     
         (R.sup.1.sub.1-x, Ba.sub.x)Ba.sub.2 Cu.sub.3 O.sub.d       (I) 
    
     wherein R 1  stands for at least one element selected from the group consisting of La, Nd, Sm, Eu and Gd, x is a number greater than 0 but not greater than 0.5 and d is a number between 6.2 and 7.2. Fine phases of RE211, RE422 and/or a metal oxide expressed by the formula (R 2   1-z , Ba z ) (Ba 1-y , R 2   y ) 2  Cu 3  O p  (R 2  =La, Nd, Sm, Eu or Gd) may be dispersed in a matrix of the matrix phase of the formula (I). The above superconductor may be obtained by cooling a melt having a temperature of 1,000°-1,300° C. and containing R 1 , Ba, Cu and O at a cooling rate of 5° C./hour or less under a partial pressure of oxygen of between 0.00001 and 0.05 atm, followed by annealing at 250°-600° C. in an oxygen atmosphere.

This application is a continuation of application Ser. No. 08/364,796filed Dec. 27, 1994, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a superconductor having a high criticaltemperature and a high critical electrical current and to a method ofproducing same.

Recent development of a melt processing technique has enabled theproduction of excellent superconductors. One such melt processingtechnique is a melt-powder-melt-growth (MPMG) process by which asuperconductor containing a Y₂ BaCuO₅ (Y211) phase dispersed in a YBa₂Cu₃ O_(y) (Y123) phase and having a high critical electrical current canbe obtained ("Melt Processed High Temperature Superconductor", M.Murakami, World Scientific, 1993). Such a superconductor can create alarge electromagnetic force upon interaction with a magnetic field andmay be utilizable for bearings, flywheels, transporting devices, etc.

Another known melt processing technique is a process in which a largecrystal, such as of Y123, having controlled crystal orientation is grownusing a seed crystal such as of La123, Nd123 or Sm123. A large crystalwith controlled crystal orientation is desirable for application tomagnetic shield and permanent magnet.

Known RE123 (RE: rare earth element) superconductors have a seriousproblem that part of the Ba site is substituted by RE ion which has alarge ionic radius so that the critical temperature is lowered (H. uweet al, Physica C, vol. 153-155, p. 930-931 (1988)).

With regard to a sintering method, there is a proposal in which a RE123superconductor is produced in a nitrogen atmosphere and the resultingproduct is subsequently oxidized so as to prevent the lowering of thecritical temperature (T. Wada et al, J. Am. Ceram. Soc., 72, 2000-2003(1989)). The superconductor thus prepared is, however, stillunsatisfactory and cannot be put into practice, because thesuperconducting transition occurs through a wide temperature range andbecause the volume fraction of the superconducting phase is small. Thisis perhaps attributed to the fact that the sintering method, whichutilizes a solid state reaction, is susceptible to an influence of thesolid solution region and solid state diffusion for homogenization of Ndon Ba sites is very sluggish so that substitution of Ba ion with RE ionsignificantly occurs.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide asuperconductor which has a high superconducting critical temperature anda high critical current and which is useful for many applications suchas magnetic levitation, magnetic shield and superconducting bulkmagnets.

Another object of the present invention is to provide a superconductorhaving a superconducting critical temperature of at least 90 K.

It is a further object of the present invention to provide a processwhich can produce a superconductor having both a high criticaltemperature and a high critical electrical current.

In accomplishing the foregoing objects, the present invention provides asuperconductor comprising a metal oxide expressed by the followingformula (I):

    (R.sup.1.sub.1-x, Ba.sub.x)Ba.sub.2 Cu.sub.3 O.sub.d       (I)

wherein R¹ stands for at least one element selected from the groupconsisting of La, Nd, Sm, Eu and Gd and x and d are numbers as follows:

    0≦x≦0.5

    6.2<d≦7.2.

In another aspect, the present invention provides a superconductorcomprising a matrix phase of a metal oxide expressed by the followingformula (I):

    (R.sup.1.sub.1-x, Ba.sub.x)Ba.sub.2 Cu.sub.3 O.sub.d       (I)

wherein R¹ stands for at least one element selected from the groupconsisting of La, Nd, Sm, Eu and Gd and x and d are numbers as follows:

    0≦x≦0.5

    6.2≦d≦7.2,

and a multiplicity of regions dispersed in said matrix phase and eachcomposed of a metal oxide expressed by the following formula (II):

    (R.sup.2.sub.1-z, Ba.sub.z)(Ba.sub.1-y, R.sup.2.sub.y).sub.2 Cu.sub.3 O.sub.p                                                   (II)

wherein R² stands for at least one element selected from the groupconsisting of La, Nd, Sm, Eu and Gd and z, y and p are numbers asfollows:

    0≦z≦0.5

    0≦y<0.5

    6.2≦p≦7.2.

In a further aspect, the present invention provides a superconductorcomprising a matrix phase of a metal oxide expressed by the followingformula (I):

    (R.sup.1.sub.1-x, Ba.sub.x)Ba.sub.2 CU.sub.3 O.sub.d       (I)

wherein R¹ stands for at least one element selected from the groupconsisting of La, Nd, Sm, Eu and Gd and x and d are numbers as follows:

    0≦x≦0.5

    6.2≦d≦7.2,

and

a multiplicity of regions dispersed in said matrix phase and eachcomposed of at least one member selected from the group consisting of(a) a metal oxide expressed by the following formula (III):

    R.sup.3.sub.4-w Ba.sub.2+w Cu.sub.2 O.sub.10-q             (III)

wherein R³ stands for at least one element selected from the groupconsisting of La and Nd and w and q are numbers as follows:

    0≦w≦0.2

    0≦q≦0.5,

and

(b) a metal oxide expressed by the following formula (IV):

    R.sup.4.sub.2 BaCuO.sub.5                                  (IV)

wherein R⁴ stands for at least one element selected from the groupconsisting of Sm, Y, Eu, Gd, Dy, Ho, Er and Yb, the amount of saidregions being not greater than 40% by weight based on the total weightof said matrix phase and said regions.

In a still further aspect, the present invention provides asuperconductor comprising a matrix phase of a metal oxide expressed bythe following formula (I):

    (R.sup.1.sub.1-x, Ba.sub.x)Ba.sub.2 Cu.sub.3 O.sub.d       (I)

wherein R¹ stands for at least one element selected from the groupconsisting of La, Nd, Sm, Eu and Gd and x and d are numbers as follows:

    0≦x≦0.5

    6.2≦d≦7.2,

a multiplicity of first regions dispersed in said matrix phase and eachcomposed of a metal oxide expressed by the following formula (II):

    (R.sup.2.sub.1-z, Ba.sub.z)(Ba.sub.1-y, R.sup.2.sub.y).sub.2 Cu.sub.3 O.sub.p                                                   (II)

wherein R² stands for at least one element selected from the groupconsisting of La, Nd, Sm, Eu and Gd and z, y and p are numbers asfollows:

    0≦z≦0.5

    0≦y≦0.5

    6.2≦p≦7.2,

and

a multiplicity of second regions dispersed in said matrix phase and eachcomposed of at least one member selected from the group consisting of(a) a metal oxide expressed by the following formula (III):

    R.sup.3.sub.4-w Ba.sub.2+w Cu.sub.2 O.sub.10-q             (III)

wherein R³ stands for at least one element selected from the groupconsisting of La and Nd and w and q are numbers as follows:

    0≦w≦0.2

    0≦q≦0.5,

and

(b) a metal oxide expressed by the following formula (IV):

    R.sup.4.sub.2 BaCuO.sub.5                                  (IV)

wherein R⁴ stands for at least one element selected from the groupconsisting of Sm, Y, Eu, Gd, Dy, Ho, Er and Yb, the amount of saidsecond regions being not greater than 40% by weight based on the totalweight of said matrix phase and said first and second regions.

The present invention also provides a process for the production of theabove superconductor, comprising providing a melt having a temperatureof 1,000°-1,300° C. and containing R¹, Ba, Cu and O in such amounts thatthe content of R¹ is 0.3-0.6 mole, the content of Ba is 0.6-0.8 mole andthe content of O is 2.1-2.7 moles each per mole of Cu, and cooling saidmelt at a cooling rate of 5° C./hour or less under a partial pressure ofoxygen of between 0.00001 and 0.05 atm, thereby to form asuperconducting phase.

FIG. 1 illustrates a ternary phase diagram of a RE₂ O₃ --BaO--CuOsystem, in which RE represents a rare earth element having a large ionicradius such as La, Nd, Sm, Eu or Gd. Because a RE_(1+x) Ba_(2-x) Cu₃O_(y) (x>0, 6.8<y<7.2) phase formed as a result of the substitution ofRE for a part of the Ba sites of a RE123 phase is as stable as the RE123phase in air, a solid solution region as shown in FIG. 1 exists. In sucha solid solution phase, the carrier concentration is reduced by thesubstitution of the trivalent ion for the divalent ion to cause thelowering of the critical temperature. The range of the solid solutionregion varies with the kind of RE. It is to be noted that RE422 phase isformed in the case where RE is La or Nd, while RE211 phase is formed inthe case where RE is Sm, Eu or Gd.

The present inventors have unexpectedly found that when a RE--Ba--Cu--Osuperconductor is produced from a melt while maintaining the oxygenpartial pressure in a range of 0.00001-0.05 atm, substitution of RE forpart of Ba site is prevented and, rather, Ba ion substitutes for a partof the RE ion. As a consequence the resulting superconductor has a highcritical temperature. In FIG. 1, designated as P is the range of solidsolution in the present invention, while Q designates the range of solidsolution when processed in air.

It has also been found that the formation of dispersed phases eachformed of a metal oxide of the above formula (II) in a matrix phase of ametal oxide of the above formula (I) gives a pinning effect superior tothat attained in the known system containing Y211 phases dispersed in aY123 matrix phase, so that the superconductor shows a high criticalcurrent even in a strong magnetic field. Each of the dispersed phasespreferably has a diameter of not greater than 1 μm.

The presence of dispersed phases of a metal oxide of the above formula(III) or (IV) in an amount of not greater than 40% by volume has alsobeen found to be effective in obtaining a pinning effect. The 211 or 422phase generally has an average diameter of not greater than 30 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome apparent from the detailed description of the preferredembodiments of the invention which follows, when considered in light ofthe accompanying drawings, in which:

FIG. 1 is a ternary phase diagram of a RE₂ O₃ --BaO--CuO system, inwhich RE represents a rare earth element having a large ionic radius;

FIG. 2 shows X-ray photoelectron spectra of Ba 3d core electron of BaOand a Nd--Ba--Cu--O superconductor;

FIG. 3 is a diagram showing the crystal structure of a superconductoraccording to the present invention having a formula (R¹ _(-x),Ba_(x))Ba₂ Cu₃ O_(d) ;

FIGS. 4 and 4A are graphs showing a relationship between the electricalresistivity and the temperature of a superconductor obtained in Example1;

FIGS. 5A and 5B are diagrams showing the results of analysis of thesuperconductor of Example 1 by an electron probe microanalyzer;

FIG. 6 is a graph showing a relationship between the irreversibilityfield and the temperature of the superconductor of Example 1 and a knownsuperconductor produced by the MPMG method;

FIG. 7 is a graph showing a relationship between the pinning force andthe magnetic field of the superconductor of Example 1 and the knownsuperconductor; and

FIG. 8 is a graph showing a relationship between the critical currentdensity and the magnetic filed of a superconductors obtained in Example8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The superconductor according to the present invention may be produced bya process in which a melt containing R¹, Ba, Cu and O and preferablyhaving a content of R¹ of 0.3-0.6 mole, a content of Ba of 0.6-0.8 moleand a content of O of 2.1-2.7 moles each per mole of Cu is cooled undera partial pressure of oxygen of between 0.00001 and 0.05 atm.

Any known melt processed method may be adopted for the purpose of thepresent invention so long as a metal oxide superconductor is grown froma melt. Thus, a method in which a single crystal superconductor is grownon a seed crystal in a melt may be adopted if desired. Oxides, organicor inorganic salts of respective metals may be generally used as rawmaterials for the formation of the melt.

It is important that the formation and growth of the crystal beperformed under the above-specified oxygen partial pressure. An oxygenpartial pressure below 0.00001 atm is disadvantageous because thesuperconducting phase is apt to be decomposed. When the oxygen partialpressure exceeds 0.05 atm, the substitution of Ba ion for part of the REsites fails to occur satisfactorily. Preferably, the crystal growth isperformed in the atmosphere of an inert gas such as argon or nitrogenwhile controlling the oxygen partial pressure as above.

It is also important that the melt should initially have a temperatureof 1,000° C. to 1,300° C. since it is difficult to maintain the moltenstate when the temperature is lower than 1,000° C. and since the melttends to react with a supporting member, such as a vessel, in which themelt is received when the temperature exceeds 1,300° C.

The cooling rate should not be greater than 5° C. per hour sinceotherwise it is difficult to grow the crystal in a stable manner. It ispreferred that the cooling of the melt be performed while maintaining atemperature gradient of 5° C./cm or more in a direction along which thecrystal grows.

It is also preferred that the cooling step be followed by annealing inan atmosphere of oxygen. The annealing is generally performed at atemperature of 250°-600° C., preferably 300°-400° C. for a period of atleast 10 hours, preferably 10 hours to 10 days.

The following examples will further illustrate the present invention.

EXAMPLE 1

Powders of Nd₂ O₃, BaCO₃ and CuO were mixed with each other with a ratioof Nd:Ba:Cu of 1:2:3. The mixture was calcined at 900° C. for 24 hoursand placed in a platinum crucible. The contents in the crucible werethen heated and fused at 1,400° C. for 20 minutes. The crucible wasgrasped with copper hammers and rapidly cooled. The cooled mass waspulverized and then shaped into a plate-like sample of a 50mm×50mm×20mmsize. Then, while being maintained under an oxygen partial pressure of0.01 atm, the sample was heated to 1,120° C., maintained at thattemperature for 20 minutes, cooled to 1,080° C. in 20 minutes andgradually cooled to 950° C. at a rate of 1° C./hour. This was furthertreated at 350° C. for 100 hours under an oxygen pressure of 1 atm toobtain a Nd--Ba--Cu--O superconductor according to the presentinvention.

FIG. 2 shows a result of the measurement of the binding energy of 3dcore electron of Ba of the Nd--Ba--Cu--O superconductor (Spectrum 1).The upper curve (Spectrum 2) shows a result of Bao and represents aresult of divalent Ba ion. As seen from FIG. 2, the binding energy of Ba3d core electron is shifted toward the low energy side, suggesting thata part of the Nd site is substituted by Ba ion. FIG. 3 schematicallyillustrates the crystal structure of the superconductor of the presentinvention.

As a result of the substitution of divalent Ba ion for a part oftrivalent Nd ion, the hole concentration is increased so that thecritical temperature is raised. FIG. 4 shows temperature dependence ofthe electrical resistivity of the above Nd--Ba--Cu--O superconductor. Aswill be appreciated from the result of FIG. 4, the superconductingtransition occurs at a surprisingly very high temperature.

FIG. 5 shows the results of a compositional analysis of theNd--Ba--Cu--O superconductor with an electron probe microanalyzer. Theanalysis reveals that the matrix phase has a chemical formula of Nd₀.9Ba₂.1 Cu₃.0 O₇.0, while the dispersed regions have a phase with asmaller Ba/Nd ratio. More detailed electron probe microanalysis using atransmission electron microscope revealed that the dispersed phases havethe formula represented by (Nd_(1-z), Ba_(z))(Ba_(1-y), Nd_(y))₂ Cu₃O_(p) wherein z is a number greater than 0 but smaller than 0.5, y is anumber greater than 0 but smaller than 0.5 and p is a number greaterthan 6.8 but smaller than 7.2. The dispersed regions each having adiameter of less than 1 μm show characteristics, such as criticaltemperature and critical current, different from those of the matrix. Inparticular, the superconductivity is destroyed under a strong magneticfield to cause transition into the normal conductivity. Since theintensity of the magnetic field which causes the normal conductivitytransition varies with the composition of the dispersed regions, thesedispersed regions can function as pinning centers in a wide range of themagnetic field.

As a consequence of the presence of the dispersed phases with variouscompositions, the Nd--Ba--Cu--O superconductor according to the presentinvention gives a high irreversibility field as will be appreciated fromFIG. 6. Since the maximum magnetic field above which a superconductor isnot actually usable is determined by the irreversibility field thereof,the high irreversibility field attained in the Nd--Ba--Cu--Osuperconductor has a great significance.

Comparative Example 1

Example 1 was repeated in the same manner as described except that themelt grow process was performed in air. The analysis with an electronbeam microanalyzer reveals that the resulting Nd--Ba--Cu--Osuperconductor has a formula: Nd₁.1 Ba₁.9 Cu₃.0 O₆.9. Thissuperconductor has a superconducting transition temperature of 85 K.

Comparative Example 2

Powders of Y₂ O₃, BaCO₃ and CuO were mixed with each other with a ratioof Y:Ba:Cu of 1.8:2.4:3.4. The mixture was calcined at 900° C. for 24hours and placed in a platinum crucible. The contents in the cruciblewere then heated and fused at 1,400° C. for 20 minutes. The crucible wasgrasped with copper hammers and rapidly cooled. The cooled plate waspulverized and then shaped into a plate-like sample of a 50mm×50mm×20mmsize. Then, the sample was heated to 1,120° C., maintained at thattemperature for 20 minutes, cooled to 1,080° C. in 20 minutes andgradually cooled to 950° C. at a rate of 1° C./hour in air. This wasfurther treated at 500° C. for 100 hours under an oxygen pressure of 1atm to obtain a Y--Ba--Cu--O superconductor containing fine 211 phasesdispersed in a Y123 matrix phase. The irreversibility curve and themagnetic flux pinning curve of the Y--Ba--Cu--O superconductor are shownin FIGS. 6 and 7. As compared with the Y--Ba--Cu--O superconductorobtained by the conventional melt processing method, the Nd--Ba--Cu--Osuperconductor of the present invention shows a much higherirreversibility field (FIG. 6) and a higher pinning force (FIG. 7).

EXAMPLE 2

Powders of Nd₂ O₃, BaCO₃ and CuO were mixed with each other with a ratioof Nd:Ba:Cu of (A) 1.2:2.1:3.1, (B) 1.4:2.2:3.2 and (C) 1.8:2.4:3.4.Each of the mixtures was processed in the same manner as described inExample 1 to obtain three plate-like samples. Then, while beingmaintained under an oxygen partial pressure of 0.001 atm, each of thesamples was heated to 1,120° C., maintained at that temperature for 20minutes, cooled to 1,080° C. in 20 minutes and gradually cooled to 950°C. at a rate of 1° C./hour. This was further treated at 350° C. for 100hours under an oxygen pressure of 1 atm to obtain three Nd--Ba--Cu--Osuperconductors (A)-(C) according to the present invention. Thesuperconductors (A)-(C) were found to contain Nd422 phases havingformulas of Nd₃.9 Ba₂.1 Cu₂ O₉.6, Nd₃.85 Ba₂.15 Cu₂ O₉.7 and Nd₃.95Ba₂.05 Cu₂ O₉.6, respectively dispersed in a matrix phase in an amountof about 9%, 16% and 29% by volume, respectively, and to havesuperconducting critical temperature of 93 K., 95 K. and 94 K.,respectively.

EXAMPLE 3

Powders of La₂ O₃, BaCO₃ and CuO were mixed with each other with a ratioof La:Ba:Cu of 1.2:2.1:3.1 and the mixture was processed in the samemanner as described in Example 1 to obtain a plate-like sample. Then,while being maintained under an oxygen partial pressure of 0.01 atm, thesample was heated to 1,120° C., maintained at that temperature for 20minutes, cooled to 1,080° C. in 20 minutes and gradually cooled to 900°C. at a rate of 1° C./hour. This was further treated at 350° C. for 100hours under an oxygen pressure of 1 atm to obtain a La--Ba--Cu--Osuperconductor according to the present invention having asuperconducting critical temperature of 90 K. For the purpose ofcomparison, the above procedure was repeated in the same manner asdescribed except that the melt grow process was performed in air. Theresulting superconductor had a critical temperature of 10 K.

EXAMPLE 4

Powders of Sm₂ O₃, BaCO₃ and CuO were mixed with each other with a ratioof Sm:Ba:Cu of 1.2:2.1:3.1 and the mixture was processed in the samemanner as described in Example 1 to obtain a plate-like sample. Then,while being maintained under an oxygen partial pressure of 0.01 atm, thesample was heated to 1,120° C., maintained at that temperature for 20minutes, cooled to 1,060° C. in 20 minutes and gradually cooled to 900°C. at a rate of 1° C./hour. This was further treated at 350° C. for 100hours under an oxygen pressure of 1 atm to obtain a Sm--Ba--Cu--Osuperconductor according to the present invention having asuperconducting critical temperature of 92 K. For the purpose ofcomparison, the above procedure was repeated in the same manner asdescribed except that the treatment of the melt grow process wasperformed in air. The resulting superconductor had a criticaltemperature of 65 K.

EXAMPLE 5

Powders of Eu₂ O₃, BaCO₃ and CuO were mixed with each other with a ratioof Eu:Ba:Cu of 1.2:2.1:3.1 and the mixture was processed in the samemanner as described in Example 1 to obtain a plate-like sample. Then,while being maintained under an oxygen partial pressure of 0.01 atm, thesample was heated to 1,120° C., maintained at that temperature for 20minutes, cooled to 1,040° C. in 20 minutes and gradually cooled to 900°C. at a rate of 1° C./hour. This was further treated at 350° C. for 100hours under an oxygen pressure of 1 atm to obtain a Eu--Ba--Cu--Osuperconductor according to the present invention having asuperconducting critical temperature of 92 K. For the purpose ofcomparison, the above procedure was repeated in the same manner asdescribed except that the melt grow process was performed in air. Theresulting superconductor had a critical temperature 75 K.

EXAMPLE 6

Powders of Gd₂ o3, BaCO₃ and CuO were mixed with each other with a ratioof Gd:Ba:Cu of 1.2:2.1:3.1 and the mixture was processed in the samemanner as described in Example 1 to obtain a plate-like sample. Then,while being maintained under an oxygen partial pressure of 0.01 atm, thesample was heated to 1,120° C., maintained at that temperature for 20minutes, cooled to 1,020° C. in 20 minutes and gradually cooled to 900°C. at a rate of 1° C./hour. This was further treated at 350° C. for 100hours under an oxygen pressure of 1 atm to obtain a Gd--Ba--Cu--Osuperconductor according to the present invention having asuperconducting critical temperature of 93 K. For the purpose ofcomparison, the above procedure was repeated in the same manner asdescribed except that the treatment of the melt grow process wasperformed in air. The resulting superconductor had a criticaltemperature of 80 K.

EXAMPLE 7

Powders of Sm₂ O₃, BaCO₃ and CuO were mixed with each other with a ratioof Sm:Ba:Cu of 1:2:3 and the mixture was processed in the same manner asdescribed in Example 1 to obtain a plate-like sample. Then, while beingmaintained under an oxygen partial pressure of 0.01 atm, the sample washeated to 1,120° C., maintained at that temperature for 20 minutes,cooled to 1,080° C. in 20 minutes and gradually cooled to 900° C. at arate of 5° C./hour. This was further treated at 350° C. for 100 hoursunder an oxygen pressure of 1 atm to obtain a Sm--Ba--Cu--Osuperconductor according to the present invention having asuperconducting critical temperature of 92 K. For the purpose ofcomparison, the above procedure was repeated in the same manner asdescribed except that the cooling rate was increased to 10° C./hour. Theresulting superconductor had a critical temperature of 80 K.

EXAMPLE 8

Powders of Nd₂ O₃, BaCO₃ and CuO were mixed with each other with a ratioof Nd:Ba:Cu of 1:2:3 and the mixture was calcined at 950° C. for 24hours. The calcined mixture was shaped into a pellet having a diameterof 2 cm and a height of 1 cm. Then, while being maintained under anoxygen partial pressure of 0.01 atm, the sample was heated to 1,100° C.in an electric oven, maintained at that temperature for 1 hour, cooledto 1,080° C., gradually cooled to 900° C. at a rate of 1° C./hour andallowed to be cooled to room temperature in the oven, to obtain aNd--Ba--Cu--O superconductor according to the present invention having asuperconducting critical temperature of 95 K.

EXAMPLE 9

Powders of Sm₂ O₃, BaCO₃ and CuO were mixed with each other with a ratioof Sm:Ba:Cu of 1:2:3 and the mixture was calcined at 950° C. for 24hours. The calcined mixture was shaped into a pellet having a diameterof 2 cm and a height of 1 cm. Then, while being maintained under anoxygen partial pressure of 0.01 atm, the sample was heated to 1,100° C.in an electric oven, maintained at that temperature for 1 hour, cooledto 1,080° C., gradually cooled to 900° C. at a rate of 1° C./hour andallowed to be cooled to room temperature in the oven, to obtain aSm--Ba--Cu--O superconductor according to the present invention having asuperconducting critical temperature of 92 K. The gradual cooling from1,080° C. to 900° C. was performed while establishing a temperaturegradient of 5° C./cm along the axis of the pellet. The X-ray diffractionanalysis revealed that the (001) plane of the superconductor crystal wasoriented in parallel with the direction of the temperature gradient.Similar results were obtained when the temperature gradient wasincreased to 10° C./cm and to 30° C./cm. In the absence of thetemperature gradient, the crystal orientation was not formed though acritical temperature of 92 K. was obtained.

EXAMPLE 10

Powders of Sm₂ O₃, BaCO₃ and CuO were mixed with each other with a ratioof Sm:Ba:Cu of 1:2:3. The mixture was calcined at 950° C. for 24 hoursand placed in a platinum crucible. The contents in the crucible was thenheated and fused at 1,400° C. for 20 minutes. The crucible was graspedwith copper hammers and rapidly cooled. The cooled mass was pulverizedand then shaped into a pellet having a diameter of 2 cm and a height of1 cm. Then, while being maintained under an oxygen partial pressure of0.01 atm, the sample was heated to 1,100° C., maintained at thattemperature for 1 hour, cooled to 1,060° C., gradually cooled to 900° C.at a rate of 1° C./hour and then allowed to be cooled to roomtemperature. This was further treated at 350° C. for 100 hours under anoxygen pressure of 1 atm to obtain a Sm--Ba--Cu--O superconductoraccording to the present invention having a critical temperature of 93K. The above procedure was repeated in the same manner as describedexcept that the heat treatment of the pellet was performed under anoxygen partial pressure of 0.05 atm. The superconductor thus obtainedalso showed a critical temperature of 93 K.

FIG. 8 are graphs showing a relationship between the critical currentdensity and the magnetic field of the Sm--Ba--Cu--O superconductorsobtained above. The density was measured at 77 K. using a samplevibrating sample magnetometer in which the magnetic field was inparallel with the c-axis. As seen from FIG. 8, the superconductor meltprocessed under an oxygen partial pressure of 0.001 atm gives a highercritical current density (Curve 3) as compared with that melt processedunder 0.05 atm (Curve 4).

EXAMPLE 11

Powders of Nd₂ O₃, BaCO₃ and CuO were mixed with a ratio of Nd:Ba:Cu of0.9:2.1:3.0. The mixture was calcined at 950° C. for 24 hours and placedin a platinum crucible. The contents in the crucible was then heated to1,400° C. and held for 20 minutes. The melt was then quenched usingcopper hammers. The cooled mass was pulverized and then shaped into apellet 20 mm in diameter and 10 mm in thickness. The pellet was heatedto 1,120° C., maintained at that temperature for 20 minutes, cooled to1,080° C. in 1 hour, slowly cooled to 950° C. at a rate of 1° C./hourand then cooled down to room temperature in a furnace. The abovetreatment was performed in an atmosphere containing 99.9% Ar and 0.1% O₂gas. The resulting sample was then annealed at 300° C. for 200 hours inan oxygen gas stream to obtain a superconductor having a zeroresistivity temperature of 96.5 K. The electron probe microanalysisrevealed that this sample had a composition range of Nd₀.9-0.99Ba₂.01-2.1 Cu₃ O₆.5-7.0.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all the changes which come within the meaning and rangeof equivalency of the claims are therefore intended to be embracedtherein.

What is claimed is:
 1. A superconductor comprising a crystalline metaloxide expressed by the following formula (I):

    (R.sup.1.sub.1-x, Ba.sub.x)Ba.sub.2 Cu.sub.3 O.sub.d       (I)

wherein R¹ stands for at least one trivalent element selected from thegroup consisting of La, Nd, Sm, Eu and Gd, x is a number greater than 0but not greater than 0.5 and d is a number greater than or equal to 6.2and less than or equal to 7.2, with Ba_(x) substituting at the trivalentatom site of the crystal structure, said superconductor having acritical temperature of 92 K. or greater.
 2. A superconductor comprisinga crystalline matrix phase of a metal oxide expressed by the followingformula (I):

    (R.sup.1.sub.1-x, Ba.sub.x)Ba.sub.2 Cu.sub.3 O.sub.d       (I)

wherein R¹ stands for at least one trivalent element selected from thegroup consisting of La, Nd, Sm, Eu and Gd, x is a number greater than orequal to 0 and less than or equal to 0.5 and d is a number greater thanor equal to 6.2 and less than or equal to 7.2, with Ba_(x) substitutingat the trivalent atom site of the crystal structure, and a multiplicityof regions dispersed in said matrix phase and each composed of a metalexpressed by the following formula (II):

    (R.sup.2.sub.1-z, Ba.sub.z) (Ba.sub.1-y, R.sup.2.sub.y).sub.2 Cu.sub.3 O.sub.p                                                   (II)

wherein R² stands for at least one element selected from the groupconsisting of La, Nd, Sm, Eu and Gd, z is a number greater than or equalto 0 and less than or equal to 0.5, y is a number greater than 0 andless than or equal to 0.5 and p is a number greater than or equal to 6.2and less than or equal to 7.2, with Ba_(z) substituting at the trivalentatom site of the crystal structure and R² _(y) substituting at thebarium site of the crystal structure, said superconductor having acritical temperature of 92 K. or greater.
 3. A superconductor as claimedin claim 2, wherein each of said regions has a diameter of not greaterthan 1 μm.
 4. A superconductor comprising a crystalline matrix phase ofa metal oxide expressed by the following formula (I):

    (R.sup.1.sub.1-x, Ba.sub.x)Ba.sub.2 Cu.sub.3 O.sub.d       (I)

wherein R¹ stands for at least one trivalent element selected from thegroup consisting of La, Nd, Sm, Eu and Gd, x is a number greater than 0and less than or equal to 0.5 and d is a number greater than or equal to6.2 and less than or equal to 7.2, with Ba_(x) substituting at thetrivalent atom site of the crystal structure, and a multiplicity ofregions dispersed in said matrix phase and each composed of at least onemember selected from the group consisting of (a) a metal oxide expressedby the following formula (III):

    R.sup.3.sub.4-w Ba.sub.2+w Cu.sub.2 O.sub.10-q             (III)

wherein R³ stands for at least one element selected from the groupconsisting of La and Nd, w is a number greater than or equal to 0 andless than or equal to 0.2 and q is a number greater than or equal to 0and less than or equal to 0.5 and (b) a metal oxide expressed by thefollowing formula (IV):

    R.sup.4.sub.2 BaCuO.sub.5                                  (IV)

wherein R⁴ stands for at least one element selected from the groupconsisting of Sm, Y, Eu, Gd, Dy, Ho, Er and Yb, the amount of saidregions being not greater than 40% by weight based on the total weightof said matrix phase and said regions, said superconductor having acritical temperature of 92 K. or greater.
 5. A superconductor comprisinga crystalline matrix phase of a metal oxide expressed by the followingformula (I):

    (R.sup.1.sub.1-x, Ba.sub.x)Ba.sub.2 Cu.sub.3 O.sub.d       (I)

wherein R¹ stands for at least one trivalent element selected from thegroup consisting of La, Nd, Sm, Eu and Gd, x is a number greater than orequal to 0 and less than or equal to 0.5 and d is a number greater thanor equal to 6.2 and less than or equal to 7.2, with Ba_(x) substitutingat the trivalent atom site of the crystal structure, a multiplicity offirst regions dispersed in said matrix phase and each composed of ametal oxide expressed by the following formula (II):

    (R.sup.2.sub.1-z, Ba.sub.z) (Ba.sub.1-y, R.sup.2.sub.y).sub.2 CU.sub.3 O.sub.p                                                   (II)

wherein R² stands for at least one element selected from the groupconsisting of La, Nd, Sm, Eu and Gd, z is a number greater than or equalto 0 and less than or equal to 0.5, y is a number greater than 0 butless than or equal to 0.5 and p is a number greater than or equal to 6.2and less than or equal to 7.2, with Ba_(z) substituting at the trivalentatom site of the crystal structure and R² _(y) substituting at thebarium site of the crystal structure, and a multiplicity of secondregions dispersed in said matrix phase and each composed of at least onemember selected from the group consisting of (a) a metal oxide expressedby the following formula (III):

    R.sup.3.sub.4-w Ba.sub.2+w Cu.sub.2 O.sub.10-q             (III)

wherein R³ stands for at least one element selected from the groupconsisting of La and Nd, w is a number greater than or equal to 0 andless than or equal to 0.2 and q is a number less than or equal to 0 andgreater than or equal to 0.5 and (b) a metal oxide expressed by thefollowing formula (IV):

    R.sup.4.sub.2 BaCuO.sub.5                                  (IV)

wherein R⁴ stands for at least one element selected from the groupconsisting of Sm, Y, Eu, Gd, Dy, Ho, Er and Yb, the amount of saidsecond regions being not greater than 40% by weight based on the totalweight of said matrix phase and said first and second regions, saidsuperconductor having a critical temperature of 92 K. or greater.
 6. Asuperconductor as claimed in claim 5, wherein each of said first regionshas a diameter of not greater than 1 μm.
 7. A superconductor as claimedin claim 1, produced by a process comprising:providing a melt having atemperature of 1,000°-1,300° C. and containing R¹, Ba, Cu and O in suchamounts that the content of R¹ is 0.3-0.6 mole, the content of Ba is0.6-0.8 mole and the content of O is 2.1-2.7 moles each per mole of Cu;and cooling said melt at a cooling rate of 5° C./hour or less under apartial pressure of oxygen of between 0.00001 and 0.05 atm, to therebyform a superconducting phase.
 8. A superconductor as claimed in claim 2,produced by a process comprising:providing a melt having a temperatureof 1,000°-1,300° C. and containing R¹, Ba, Cu and O in such amounts thatthe content of R¹ is 0.3-0.6 mole, the content of Ba is 0.6-0.8 mole andthe content of O is 2.1-2.7 moles each per mole of Cu; and cooling saidmelt at a cooling rate of 5° C./hour or less under a partial pressure ofoxygen of between 0.00001 and 0.05 atm, to thereby form asuperconducting phase.
 9. A superconductor as claimed in claim 4,produced by a process comprising:providing a melt having a temperatureof 1,000°-1,300° C. and containing R¹, Ba, Cu and O in such amounts thatthe content of R¹ is 0.3-0.6 mole, the content of Ba is 0.6-0.8 mole andthe content of O is 2.1-2.7 moles each per mole of Cu; and cooling saidmelt at a cooling rate of 5 C./hour or less under a partial pressure ofoxygen of between 0.00001 and 0.05 atm, to thereby form asuperconducting phase.
 10. A superconductor as claimed in claim 5,produced by a process comprising:providing a melt having a temperatureof 1,000°-1,300° C. and containing R¹, Ba, Cu and O in such amounts thatthe content of R¹ is 0.3-0.6 mole, the content of Ba is 0.6-0.8 mole andthe content of O is 2.1-2.7 moles each per mole of Cu; and cooling saidmelt at a cooling rate of 5° C./hour or less under a partial pressure ofoxygen of between 0.00001 and 0.05 atm, to thereby form asuperconducting phase.
 11. A superconductor as claimed in claim 1,wherein said crystalline metal oxide is obtained by processing in areduced atmosphere.
 12. A superconductor comprising a crystalline metaloxide expressed by the following formula (I):

    (R.sup.1.sub.1-x, Ba.sub.x)Ba.sub.2 Cu.sub.3 O.sub.d       (I)

wherein R¹ stands for at least one trivalent element selected from thegroup consisting of La, Nd, Sm, Eu and Gd, x is a number greater than 0but not greater than 0.5 and d is a number greater than or equal to 6.2and less than or equal to 7.2, with B_(x), substituting at the trivalentatom site of the crystal structure, said superconductor having acritical temperature greater than 93 K.